Assembly of components having different coefficients of thermal expansion

ABSTRACT

A component assembly includes a first component, such as an optical component, and a second component, such as a support component, having different coefficients of thermal expansion (CTEs). The component assembly also includes a spacer having a CTE matched to that of the first component, disposed between the first component and the second component. When the CTE of the first component is greater than that of the second component, the second component includes a protrusion, and the spacer includes a complementary opening for receiving the protrusion, such that a joint between the protrusion and the complementary opening is under compressive stress. The spacer also includes a mounting area for receiving the first component, and an air gap disposed between the first component and the protrusion.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an assembly of components havingdifferent coefficients of thermal expansion (CTEs), such as an opticalcomponent and a support component. More particularly, the presentinvention relates to a component assembly including a spacer.

BACKGROUND OF THE INVENTION

Free-space optical modules, such as wavelength-selective switches(WSSs), require a high thermo-mechanical stability. The centralwavelength of an optical channel must be maintained within a very narrowwavelength band over a range of operating temperatures, e.g., about 0°C. to about 70° C. Furthermore, insertion loss must be minimized overthe range of operating temperatures.

Optical components, such as lenses and prisms, are often formed ofmaterials, such as optical glasses, having a relatively low temperaturecoefficient of refractive index (dn/dT), in order to reducetemperature-dependent performance degradation. Unfortunately, low-dn/dTmaterials often have relatively high coefficients of thermal expansion(CTEs).

Typically, optical components in an optical system are joined to asupport component, such as an optical bench, by adhesive. The supportcomponent is usually formed of a material having a relatively low CTE,in order to lessen thermal expansion of the optical system. The CTEmismatch between the optical components and the support component maylead to high tensile stresses in the optical components, which are oftenrelatively brittle, and in the adhesive joints between the opticalcomponents and the support component. In some instances, the CTEmismatch may lead to catastrophic failure, such as fracture of anoptical component, upon thermal cycling, shock, or vibration. In otherinstances, the CTE mismatch may lead to performance degradation, as aresult of movement, e.g., tilt, or distortion, e.g., a change in surfacecurvature, of an optical component.

To alleviate the CTE mismatch, one or more spacers having anintermediate CTE may be disposed between an optical component and asupport component, as disclosed in U.S. Pat. No. 6,825,997 to Hubbard,et al., issued on Nov. 30, 2004, and in U.S. Pat. No. 8,292,537 toNewswander, issued on Oct. 23, 2012, for example, which are incorporatedherein by reference. In general, the spacers and the components in thesedisclosures are joined by planar adhesive joints or by fasteners.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an assembly ofcomponents having different coefficients of thermal expansion (CTEs)with a high thermo-mechanical stability.

Accordingly, the present invention relates to an assembly of componentshaving different CTEs, the assembly comprising: a first component havinga first CTE; a second component having a second CTE different from thefirst CTE; and a spacer, disposed between the first component and thesecond component, having a third CTE substantially matched to the firstCTE and different from the second CTE, the spacer including: a mountingarea for receiving the first component; one of a protrusion and acomplementary opening for receiving the protrusion, wherein the secondcomponent includes the other one of the protrusion and the complementaryopening, wherein the second component includes the protrusion and thespacer includes the complementary opening when the third CTE is greaterthan the second CTE, and wherein the spacer includes the protrusion andthe second component includes the complementary opening when the thirdCTE is less than the second CTE, such that a joint between theprotrusion and the complementary opening is under compressive stress;and an air gap, disposed between the first component and the protrusion.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in greater detail with referenceto the accompanying drawings wherein:

FIG. 1 is a cross-sectional schematic illustration of a componentassembly according to a first embodiment of the present invention;

FIG. 2A is an isometric view of the spacer in an exemplary componentassembly according to the first embodiment of the present invention;

FIG. 2B is an isometric view of the second component and the spacer inthe exemplary component assembly;

FIG. 2C is a first isometric view of the exemplary component assembly;

FIG. 2D is a second isometric view of the exemplary component assembly;

FIG. 2E is a peel-stress contour plot of the planar joint in theexemplary component assembly;

FIG. 3 is a cross-sectional schematic illustration of a componentassembly according to a second embodiment of the present invention; and

FIG. 4 is a cross-sectional schematic illustration of a componentassembly according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an assembly of components havingdifferent coefficients of thermal expansion (CTEs), such as an opticalcomponent and a support component. The component assembly reducestensile stresses arising from the CTE mismatch between the components,while providing a strong and stable attachment of the components.Consequently, the component assembly has a high thermo-mechanicalstability.

The component assembly includes a first component having a first CTE anda second component having a second CTE different from the first CTE. Thefirst CTE may be greater than or less than the second CTE. Typically,the first CTE and the second CTE differ in magnitude by about 5 ppm/°C., preferably, by at least about 3 ppm/° C. Typically, the first CTEand the second CTE differ in magnitude by at most about 10 ppm/° C. Thecomponent having the greater CTE is referred to as a high-CTE component,and the component having the lesser CTE is referred to as a low-CTEcomponent.

Often, the component assembly forms part of an optical module, such as awavelength-selective switch (WSS). In most instances, the firstcomponent is an optical component, i.e., an optical element, such as alens, a prism, a mirror, or a diffraction grating, and the secondcomponent is a support component, such as an optical bench, a platform,a package, or a housing. Typically, the optical component is formed ofan optical glass, such as N-BK7 or fused silica, having a CTE greaterthan about 6 ppm/° C. and less than about 10 ppm/° C. The supportcomponent may be formed of a metal, such as aluminum, a ferrous alloy,such as invar, a ceramic, such as alumina or aluminum nitride, or aglass, such as fused silica or pyrex, and may have a wide range of CTEs.In some instances, the first component and the second component are eachsupport components formed of different materials.

The first component and the second component are attached through aspacer disposed between the components. The spacer has a third CTEsubstantially matched to the first CTE of the first component.Accordingly, the difference between the first CTE of the first componentand the third CTE of the spacer is smaller in magnitude than both thedifference between the first CTE and the second CTE of the secondcomponent and the difference between the second CTE and the third CTE.Generally, the difference between the first CTE and the third CTE issmall enough in magnitude to avoid significant tensile stress in a jointbetween the first component, e.g., a brittle optical component, and thespacer.

The second CTE of the second component and the third CTE of the spacer,typically, differ in magnitude by about 5 ppm/° C., preferably, by atleast about 3 ppm/° C. Typically, the second CTE and the third CTEdiffer in magnitude by at most about 10 ppm/° C. The material of thespacer is chosen so that, typically, the first CTE of the firstcomponent and the third CTE of the spacer differ in magnitude by at mostabout 2 ppm/° C., preferably, by at most about 1 ppm/° C. The spacer is,typically, formed of a ferrous alloy, such as alloy-48 or kovar.

The component assembly has different embodiments depending on whetherthe third CTE of the spacer is greater than or less than the second CTEof the second component, i.e., depending on whether the first component,to which the spacer is CTE-matched, is the high-CTE component or thelow-CTE component. In general, the spacer includes one of a protrusionand a complementary hole for receiving the protrusion, and the secondcomponent includes the other one of the protrusion and the complementaryopening. More specifically, whichever of the second component and thespacer has the lesser CTE includes the protrusion, and whichever of thesecond component and the spacer has the greater CTE includes thecomplementary opening. Advantageously, the difference between the secondCTE and the third CTE is large enough in magnitude to provide acompressive joint between the protrusion and the complementary opening.The joint is formed at an elevated temperature, and, upon cooling, thegreater contraction of the complementary opening relative to theprotrusion results in compressive stress.

In all embodiments of the component assembly, the spacer also includes amounting area for receiving the first component, and an air gap disposedbetween the first component and the protrusion. The air gap serves tofurther isolate, i.e., decouple, the first component from the secondcomponent. Thereby, thermo-mechanical distortion of the first componentresulting from the CTE mismatch between the first component and thesecond component is minimized, improving the thermo-mechanical stabilityof the component assembly.

Certain embodiments of the component assembly are described in furtherdetail hereafter. However, the present invention is not limited to theseembodiments, which are provided by way of example only.

With reference to FIG. 1, a first embodiment of the component assembly100 includes a high-CTE first component 110, a low-CTE second component120, and a high-CTE spacer 130. Both the first CTE of the firstcomponent 110 and the third CTE of the spacer 130 are greater than thesecond CTE of the second component 120.

For example, the first component 110 may be an optical element formed ofN-BK7 glass having a CTE of about 7.1 ppm/° C., the second component 120may be an optical bench formed of invar having a CTE of about 1.3 ppm/°C., and the spacer 130 may be formed of alloy-48 having a CTE of about8.5 ppm/° C. For another example, the first component 110 may be apackage formed of a ceramic having a CTE of about 7.1 ppm/° C., thesecond component 120 may be an optical bench formed of invar having aCTE of about 1.3 ppm/° C., and the spacer 130 may be formed of kovarhaving a CTE of about 6 ppm/° C.

The low-CTE second component 120 includes a protrusion 121, and thehigh-CTE spacer 130 includes a complementary opening 131 for receivingthe protrusion 121. The protrusion 121 and the complementary opening 131have forms suitable for mating with one another. Preferably, theprotrusion 121 has an elliptical or race-track-shaped cross section tofacilitate rotational alignment during assembly. Accordingly, thecomplementary opening 131, preferably, has a corresponding elliptical orrace-track-shaped cross section. Alternatively, the protrusion 121 andthe complementary opening 131 may each have a circular cross section ora cross section of another suitable shape. Typically, the protrusion 121and the complementary opening 131 are each substantially uniform incross section, i.e., substantially straight rather than tapered. Theprotrusion 121 extends upwards, i.e., vertically, from an upper surfaceof the second component 120, and the complementary opening 131 extendsupwards from a lower surface of the spacer 130.

Lateral surfaces of the protrusion 121 and the complementary opening 131are joined by adhesive, forming a ring joint 122, e.g., with a thicknessof about 50 μm. Preferably, the adhesive is a rigid heat-cure adhesivehaving a curing temperature of greater than about 100° C. As mentionedheretofore, the ring joint 122 is formed at an elevated temperature,e.g., about 100° C., so that, once cooled, the ring joint 122 is undercompressive stress in the radial and circumferential directions. Thering joint 122 remains under compressive stress as long as thetemperature of the component assembly 100 remains lower than thetemperature at which the ring joint 122 was formed, i.e., the attachmenttemperature. Accordingly, the attachment temperature, and likewise thecuring temperature of the adhesive, should be higher than usualassembly, operational, and storage temperatures of the componentassembly 100, e.g., about 0° C. to about 70° C. Advantageously, the ringjoint 122 has a relatively large bond size, which increases themechanical strength and stability of the ring joint 122.

The spacer 130 also includes a mounting area 132 for receiving the firstcomponent 110, and an air gap 133. The air gap 133 is disposed withinthe spacer 130 between the first component 110 and the protrusion 121.Typically, the air gap 133 has a vertical extent, i.e., a height, ofabout 0.5 mm to about 1 mm.

In the first embodiment, the air gap 133 is integral with thecomplementary opening 131. Together, the complementary opening 131 andthe air gap 133 form a covered hole, e.g., a blind hole. The air gap 133separates an upper surface of the protrusion 121 from the spacer 130.

The mounting area 132 is disposed on an upper surface of the spacer 130.Typically, the mounting area 132 is planar. The mounting area 132 isjoined to a lower surface of the first component 110 by adhesive, e.g.,an epoxy, forming a planar joint 111, e.g., with a thickness of about 50μm.

In the first embodiment, the mounting area 132 is disposed over thecovered hole, i.e., the air gap 133 and the complementary opening 131,and the protrusion 121. Typically, the planar joint 111 is circular,elliptical, or race-track shaped. Advantageously, the planar joint 111is under a favorable state of stress because of the out-of-planeconstraint provided by the protrusion 121, i.e., because the edges ofthe spacer 130 deform up slightly after the ring joint 122 is formed.Typically, the planar joint 111 is under compressive peel stress, whichmay help to prevent delamination, for example. Preferably, the planarjoint 111 extends laterally beyond the covered hole and the protrusion,i.e., the length of the planar joint 111 is greater than the width ofthe covered hole and the width of the protrusion 121, to maximize thecompressive peel stress. However, the lateral extent, i.e., the length,of the planar joint 111 should not be so large that shear stress becomesexcessive.

With reference to FIG. 2, an exemplary component assembly 200 accordingto the first embodiment includes an optical element as the high-CTEfirst component 210, an optical bench as the low-CTE second component220, and a high-CTE spacer 230.

With particular reference to FIG. 2A, the spacer 230 includes a body 234and a mounting bar 235. The spacer 230 also includes a complementaryopening 231, formed in the body 234, and an air gap 233, formed in themounting bar 235, which together form a covered hole. The mounting bar235 extends laterally across the hole, partially covering the hole.Advantageously, this design allows the complementary opening 231 to beaccessed from above, facilitating adhesive application and verification.A mounting area 232 is disposed on an upper surface of the mounting bar235.

With particular reference to FIG. 2B, the spacer 230 is shaped to fitinto a recess 223 on the second component 220. A race-track-shapedprotrusion 221 within the recess 223 fits into the complementary opening231 on the spacer 230, which is also race-track-shaped. Lateral surfacesof the protrusion 221 and the complementary opening 231 are joined byadhesive, forming a ring joint that is under compressive stress.

With particular reference to FIGS. 2A and 2C-2E, the first component 210is mounted on the mounting bar 235 of the spacer 230, more specifically,on the mounting area 232 on the upper surface thereof. The mounting area232 is joined to a lower surface of the first component 210 by adhesive,forming a planar joint 211. The planar joint 211 is race-track-shapedand extends laterally beyond the covered hole and the protrusion, i.e.,the length of the planar joint 211 is greater than the width of thecovered hole and the width of the protrusion 221. In FIG. 2E, apeel-stress contour plot for the planar joint 211 shows that the peelstress on the planar joint 211 is substantially compressive, asexplained heretofore.

With reference to FIG. 3, a second embodiment of the component assembly300 is similar to the first embodiment, but includes a spacer 330 havinga different design. Aspects of the second embodiment that are differentfrom the first embodiment are described hereafter.

The spacer 330 includes a complementary opening 331, a mounting area332, and an air gap 333. As in the first embodiment, the air gap 333 isintegral with the complementary opening 331. However, in the secondembodiment, the complementary opening 331 and the air gap 333 togetherform a through hole, which extends vertically through the spacer 330,i.e., from a lower surface of the spacer 330 to an upper surface of thespacer 330. The air gap 333 separates the upper surface of theprotrusion 121 from the lower surface of first component 110.

As in the first embodiment, the mounting area 332 on the upper surfaceof the spacer 330 is joined to the lower surface of the first component110 by adhesive, forming a planar joint 311. However, in the secondembodiment, the mounting area 332 is not disposed over the through hole,i.e., the air gap 333 and the complementary opening 331, and theprotrusion 121, but rather surrounds the through hole. In other words,the mounting area 332 is annular. Accordingly, the planar joint 311 isalso annular. Therefore, the mechanical strength and stability of theplanar joint 311 may be reduced relative to the first embodiment.Nevertheless, the simpler design of the second embodiment may bedesirable in some instances.

With reference to FIG. 4, a third embodiment of the component assembly400 is similar to the first embodiment, but includes a low-CTE firstcomponent 410, a high-CTE second component 420, and a low-CTE spacer 430having a different design. Aspects of the third embodiment that aredifferent from the first embodiment are described hereafter.

Both the first CTE of the first component 410 and the third CTE of thespacer 430 are less than the second CTE of the second component 420. Forexample, the first component 410 may be an optical element formed ofN-BK7 glass having a CTE of about 7.1 ppm/° C., the second component 420may be an optical bench formed of low-cost aluminum having a CTE ofabout 23 ppm/° C., and the spacer 430 may be formed of alloy-48 having aCTE of about 8.5 ppm/° C.

In the third embodiment, the spacer 430 includes a protrusion 436, andthe second component 420 includes a complementary opening 424 forreceiving the protrusion 436. The protrusion 436 extends downwards froma lower surface of the spacer 430, and the complementary opening 424extends downwards from an upper surface of the second component 420. Thecomplementary opening 424 is, typically, a blind hole, but may also be athrough hole in some instances. As in the first embodiment, lateralsurfaces of the protrusion 436 and the complementary opening 424 arejoined by adhesive, forming a ring joint 422 that is under compressivestress.

The spacer 430 also includes a mounting area 432 and an air gap 433. Theair gap 433 is disposed within the spacer 430 between the firstcomponent 410 and the protrusion 436. In the third embodiment, the airgap 433 is separate from the complementary opening 424. Typically, theair gap 433 is a slit extending laterally through the spacer 430, i.e.,between opposite lateral surfaces of the spacer 430.

As in the first embodiment, the mounting area 432 on an upper surface ofthe spacer 430 is joined to a lower surface of the first component 410by adhesive, forming a planar joint 411. In the third embodiment, themounting area 432 is disposed over the air gap 433 and the protrusion436. Typically, the planar joint 411 is circular, elliptical, orrace-track shaped. Preferably, the planar joint 411 extends laterallybeyond the air gap 433 and the protrusion 436, i.e., the length of theplanar joint 411 is greater than the width of the slit-shaped air gap433 and the width of the protrusion 436.

In general, the various embodiments of the component assembly arefabricated by the following method. Adhesive is applied to the lateralsurfaces of the protrusion and/or the complementary opening, theprotrusion is inserted into the complementary opening, and the adhesiveis heat-cured at an elevated temperature, e.g., about 100° C., to formthe ring joint between the second component and the spacer. Adhesive isthen applied to the mounting area on the upper surface of the spacer,the lower surface of the first component is mounted on the mountingarea, and the adhesive is cured, typically, at an elevated temperature,e.g., about 100° C., or, in some instances, at a lower temperature, toform the planar joint between the first component and the spacer.

Of course, numerous other embodiments may be envisaged without departingfrom the spirit and scope of the invention.

I claim:
 1. An assembly of components having different coefficients ofthermal expansion (CTEs), the assembly comprising: a first componenthaving a first CTE; a second component having a second CTE differentfrom the first CTE; and a spacer, disposed between the first componentand the second component, having a third CTE substantially matched tothe first CTE and different from the second CTE, the spacer including: amounting area for receiving the first component; one of a protrusion anda complementary opening for receiving the protrusion, wherein the secondcomponent includes the other one of the protrusion and the complementaryopening, wherein the second component includes the protrusion and thespacer includes the complementary opening when the third CTE is greaterthan the second CTE, and wherein the spacer includes the protrusion andthe second component includes the complementary opening when the thirdCTE is less than the second CTE, such that a joint between theprotrusion and the complementary opening is under compressive stress;and an air gap, disposed between the first component and the protrusion.2. The assembly of claim 1, wherein the mounting area is disposed on anupper surface of the spacer, and wherein the one of the protrusion andthe complementary opening extends from a lower surface of the spacer. 3.The assembly of claim 2, wherein the mounting area and a lower surfaceof the first component are joined by adhesive, and wherein lateralsurfaces of the protrusion and the complementary opening are joined byadhesive.
 4. The assembly of claim 2, wherein the mounting area isdisposed over the air gap and the protrusion.
 5. The assembly of claim4, wherein the mounting area extends laterally beyond the air gap andthe protrusion.
 6. The assembly of claim 4, wherein a joint between themounting area and the first component is under compressive peel stress.7. The assembly of claim 1, wherein the joint between the protrusion andthe complementary hole is a ring joint, and wherein a joint between themounting area and the first component is a planar joint.
 8. The assemblyof claim 1, wherein the mounting area is planar.
 9. The assembly ofclaim 1, wherein the protrusion has an elliptical or race-track-shapedcross section, and wherein the complementary hole has a correspondingelliptical or race-track-shaped cross section.
 10. The assembly of claim1, wherein the first component is an optical component, and wherein thesecond component is a support component.
 11. The assembly of claim 1,wherein a difference between the first CTE and the third CTE is smallerin magnitude than both a difference between the first CTE and the secondCTE and a difference between the second CTE and the third CTE.
 12. Theassembly of claim 1, wherein the third CTE is greater than the secondCTE, wherein the second component includes the protrusion, and whereinthe spacer includes the complementary opening.
 13. The assembly of claim12, wherein the air gap and the complementary opening together form acovered hole.
 14. The assembly of claim 13, wherein the covered holeextends upwards from a lower surface of the spacer, and wherein themounting area is disposed on an upper surface of the spacer over thecovered hole and the protrusion.
 15. The assembly of claim 14, whereinthe spacer further includes a mounting bar extending across andpartially covering the covered hole, and wherein the mounting area isdisposed on an upper surface of the mounting bar.
 16. The assembly ofclaim 12, wherein the air gap and the complementary opening togetherform a through hole.
 17. The assembly of claim 16, wherein the throughhole extends from a lower surface of the spacer to an upper surface ofthe spacer, and wherein the mounting area is disposed on the uppersurface of the spacer surrounding the through hole.
 18. The assembly ofclaim 1, wherein the third CTE is less than the second CTE, wherein thespacer includes the protrusion, and wherein the second componentincludes the complementary opening.
 19. The assembly of claim 18,wherein the air gap is a slit.
 20. The assembly of claim 19, wherein theprotrusion extends downwards from a lower surface of the spacer, whereinthe slit extends laterally through the spacer, and wherein the mountingarea is disposed on an upper surface of the spacer over the slit and theprotrusion.